1. Field of the Invention
The present invention relates to medical instruments and the field of cardiac electrophysiology, and more particularly to the localization of foci of abnormal cardiac activation by comparative analysis of body surface potential distributions during cardiac pacing.
2. Description of the Related Art
The contraction of the human heart is triggered by an electrical process known as an "action potential" which depolarizes the transmembrane potential of the myocardial cells. This depolarization process is automatic (i.e. an isolated cardiac cell can show repeated action potentials) and also, it propagates from one cell to the neighboring cells. In the normal heart, activation originates from the cells having the fastest ation potential frequency which are located in the sino-atrial (SA) node. The activation propagates from the SA node to the rest of the atria, and then to the ventricles through the atrioventricular (AV) node, which slows down propagation so as to permit the flow of blood from the atria to the ventricles, and then through the His bundle and the Purkinje conduction system which synchronizes the ventricular activation.
Congenital heart diseases or complications following coronary artery disease can produce an abnormal increase of the heart rate known as tachycardia which can be potentially lethal. In patients with the Wolff-Parkinson-White (WPW) syndrome (a congenital disease), an additional conduction pathway joins the atria and the ventricles and this accessory AV pathway can either be responsible for the continuous activation of the ventricles during atrial tachyarrhythmias, or create a reentrant circuit in which activation propagates repeatedly through the ventricles, the accessory AV pathway, the atria, the AV node and the ventricles again. In patients with idiopathic ventricular tachycardia or in patients with ventricular tachycardia (VT) resulting from myocardial infarction, the abnormal activation originates during VT from a circumscribed region of the ventricles with abnormal automaticity and/or propagation properties.
These disorders of the heart rhythm may be cured by the catheter ablation of the arrhythmogenic sites, either the accessory AV pathways or the sites of origin of VT. See: W. M. Jackman et al., "Catheter ablation of accessory atrioventricular pathways, Wolff-Parkinson-White syndrome, by radiofrequency current", New Engl J Med 324:1605, 1991; L. S. Klein et al., "Radiofrequency catheter ablation of ventricular tachycardia in patients without structural heart disease", Circulation 85:1666, 1992. Catheter ablation consists of inserting a catheter percutaneously through veins or arteries inside the heart cavities. The tip of the catheter is placed near the arrhythmogenic site. Electromagnetic energy is then delivered to the myocardium by electrodes or antennas located at the catheter tip. Electromagnetic energy can be used within a wide frequency spectrum ranging from DC current to radiofrequency current, microwave and laser light. The effects of this localized energy delivery is to destroy the arrhythmogenic site and to create a small permanent lesion.
One of the problems with the catheter ablation of arrhythmogenic sites is the duration of the procedure. This procedure is long because cardiologists rely on electrograms recorded with electrodes located near the catheter tip to guide the positioning of the catheter within millimeters of the arrhythmogenic site. On these electrograms, the timing of the local activation deflexion or the presence of accessory pathway potentials can only indicate if the catheter tip is near or far from the arrhythmogenic site, and if it is far, it does not indicate in which direction to move the catheter.
Information about the location of the arrhythmogenic site can be obtained from body surface potential maps (BSPM). As ventricular activation progresses away from the accessory AV pathway or from the VT site of origin, the activation currents generate electrical potentials that can be measured over the torso surface by a large number of electrodes. For WPW patients, Benson et al. (Benson et al., "Localization of the site of ventricular preexcitation with body surface potential maps in patients with the Wolff-Parkinson-White syndrome", Circulation, 65:1259, 1982) correlated the patterns of BSPM recorded during the preexcitation of the ventricles through the accessory AV pathway (delta wave) with the preexcitation sites determined by electrophysiologic studies or surgical ablations and they concluded that at least seven preexcitation sites could be predicted by analysis of the BSPM patterns. Similar patterns were reported by Nadeau et al. (Nadeau et al. "Localization of preexcitation sites in the Wolff-Parkinson-White syndrome by body surface potential mapping and a single moving dipole representation". In Electrocardiographic body surface potential mapping Eds R. T. van Dam, A. van Oosterom, Martinus Nijhoff, pp: 95-98, 1986) in patients who underwent arrhythmia surgery and/or an electrophysiologic study. The latter also noted the progressive changes in the morphology of the BSPM recorded during the delta wave in patients with adjacent preexcitation sites, thus reflecting the continuous distribution of possible accessory AV pathways around the AV ring with the position of the minimum and of negative potentials on the BSPM identifying the pathway location: prominent negativity on the right side of the anterior torso correspond to patients with preexcitation sites located in the right ventricle; a minimum on the back correspond to sites in the left ventricle; negativity over the entire lower torso correspond to posteroseptal sites; otherwise, positivity over the entire lower torso correspond to anterior sites. Similarly, for the localization of sites of origin of VT, Sippens-Groenewegen et al. (Sippens-Groenewegen et al., "Body surface mapping of ectopic left and right ventricular activation. QRS spectrum in patients without structural heart disease", Circulation, 82:879, 1990) reported BSPM patterns obtained during ventricular pacing at known sites and which can be used to estimate the site of origin of ectopic activity.
Another approach to the localization of abnormal cardiac activation which uses electrocardiographic potentials consists of comparing the standard twelve-lead electrocardiogram (ECG) during ventricular pacing at different sites, with the ECG recorded during abnormal ventricular activation (pacing consists of initiating the activation process of the ventricles by applying a small current pulse between the electrodes of a catheter located inside the ventricles). This "pace-mapping" approach relies on the visual analysis of twelve time-varying signals. It can confirm that the pacing catheter is located over the focus of abnormal activation when the paced ECG and the abnormal ECG are identical (because the cardiac activation that generates these two ECGs are localized at the same site). However, it gives only limited information about which direction to move the catheter toward the focus when the two ECG are not identical. See: Josephson et al., "Ventricular activation during ventricular endocardial pacing. II. Role of pace mapping to localize origin of ventricular tachycardia", Am J Cardiol 50:11, 1982.
The Patent literature also provides teachings which are of interest having regards to the invention as described and claimed in the following, for example, U.S. Pat. No. 4,974,598, John, Dec. 4, 1990, U.S. Pat. No. 5,083,565, Parins, Jan. 28, 1992, U.S. Pat. No. 5,069,215, Jadvar et al, Dec. 3, 1991, U.S. Pat. No. 4,751,931, Briller et al, Jun. 21, 1988 and U.S. Pat. No. 4,641,649, Walinsky et al, Feb. 10, 1987.
The teachings of the '598 patent relate to early detection of heart disease with an EKG system which detects heart beats having P, Q, R, S, T and U portions. A large number of electrodes (32 to 64) are placed on the torso of the patient. Readings taken are subjected to statistical analysis and compared with readings of a normal population.
In the '565 patent, an electrosurgical catheter includes a sensor for sensing the polarization signals developed in the heart and for transmitting the sensed signals to an external EKG monitor. It also includes insulated tips to which an RF signal may be applied to destroy selected cells.
A disposable esophageal electrode structure, as taught in the '215 patent, includes a plurality of spaced apart conductive electrode members. Each electrode is connected to a wire by which it is connected to external electrical units.
In the '931 patent, surface electrodes are positioned on the surface of the patient's body in the heart area of the patient. Surface ECG's acquired are enhanced by filtering and then subjected to method steps for detecting low level bioelectric signals.
The '649 patent teaches a method for selective ablation of cardiac tissues by high frequency electromagnetic energy. A catheter, which is introduced into a patient's heart chamber, is terminated by an antenna. Depolarization signals are coupled by the antenna to an ECG monitor for display. External electrodes also detect potentials which are displayed on the monitor. Accordingly, the position of the antenna is adjusted to an appropriate position for ablation procedures.